The present invention pertains to a method and apparatus for drying fruits, and more particularly, to a method and apparatus that may be used to dry raisins, peaches, figs, nuts, cherries, and the like to have a moisture content of 16% or less, according to U.S. Department of Agriculture standards.
Although the present invention has application in various fruit and nut drying industries, the specific embodiment herein described relates to the application of the present invention in the grape (and, particularly, the raisin) industry. Those skilled in the art will appreciate the advantages in using the present invention in their own particular industries for reducing the moisture content of their fruits or nuts to the point where such fruits or nuts may be successfully stored without rotting.
Prior art raisin drying techniques include the steps of picking the grapes from the vine, allowing the grapes to dry in the sun for a prescribed period of time, "cigaretting" the grapes (that is, wrapping the grapes in paper much in the form of a large cigarette) and allowing the raisins so wrapped to continue to be exposed to the sunlight for a prescribed period of time, and then transferring the partially dried raisins (resulting from these grape-drying steps) to boxes called sweats which are then removed from the field. From the sweats, the partially dried raisins are transferred to trays which are loaded on cars and inserted into a dehydrator, which is much like a wind tunnel into one end of which is blown hot air. Because the end of the dehydrator into which the hot air is blown has a higher temperature than the end from which the hot air exits, regular rotation of the cars containing the trays is necessary to avoid "carmelizing" of the raisins (that is, burning them).
In the prior art technique, however, carmelizing of the raisins nevertheless is a frequent occurrence, and in the transference of the partially dried raisins from the sweats to the trays, a loss of approximately 10% of the raisins occurs. Additionally, the dehydrators of the prior art are costly to purchase, operate and time consuming.
The present invention, on the other hand, includes a method of drying raisins and apparatus therefor which was originally designed with an individual grape grower in mind. It offers the flexibility of a self-contained unit, but requires less manpower to operate. One of the reasons for this advantage is the unique air system which differs completely in principle from the conventional dehydrator-type dryer. In the method and apparatus of the present invention, temperature of an elevated temperature is moved from side to side in a drying room as opposed to the front-to-back movement in the conventional dehydrator. This creates more uniform drying within the drying room. Additionally, the method of the present invention allows the partially dried grapes or raisins to be placed within the drying room in the sweats, rather than transferring the partially dried raisins to trays. In the sweats, and because of the side-to-side movement of the air within the drying room, the raisins within the sweats dry uniformly and are all finished at one time. There is no need to rotate trays in and out of the drying room as is necessary in the prior art dehydrator method. Additionally, a large volume and high velocity of air used in the prior art dehydrator method is substantially reduced, while at the same time allowing for uniform elevated temperatures and more even air penetration of the raisins. This is accomplished within the method and apparatus of the present invention by partially reusing heated air, thus adding fuel savings to its features.
As contrasted with a standard 55 foot prior art raisin dryer, which can hold 15 cars (each car holding 25 trays of 55 pounds of raisins each), the method and apparatus for drying raisins of the present invention of a comparable size holds 30 palletts of sweats, each pallett containing 16 sweats, or a total of 35 tons of raisins. In the method and apparatus of the present invention without rotating or moving the 30 palletts of sweats, the raisins are dried to U.S.D.A. standards within 20 to 24 hours as contrasted with the necessity in the prior art to check the trays of raisins every 2 or 3 hours and rotate them at such intervals, thus accomplishing drying in approximately 24 hours on a warm day.